Most of the world's heavy water supplies are currently provided by the Girdler-Sulphide process or processes based on ammonia-hydrogen catalytic exchange. The Girdler-Sulphide process is a bithermal (two temperature) heavy water production process and takes advantage of differences in thermodynamic separation factors between water and hydrogen sulphide. The process uses a cascaded series of dual-temperature, mass transfer columns circulating large quantities of hydrogen sulphide gas countercurrently to natural water feed. Both the Girdler-Sulphide process and ammonia-hydrogen catalytic exchange require large capital expenditures. The ammonia process has size limitations and the Girdler-Sulphide process consumes large amounts of energy and utilizes very hazardous hydrogen sulphide.
The exceptional properties of heavy water as a neutron moderator make it useful in nuclear reactors and in particular the CANDU nuclear reactor developed by Atomic Energy of Canada Limited. However, the high cost of heavy water produced using the Girdler-Sulphide process and ammonia-based processes can affect the economic attractiveness of heavy-water moderated reactors such as CANDU. Accordingly, a number of other processes have been proposed for heavy water production. Among them are processes that exploit deuterium isotope exchange between water and hydrogen using a catalyst.
One such process is known as Combined Electrolysis and Catalytic Exchange (“CECE”). The CECE process has previously been described in U.S. Pat. No. 3,974,048. It is a monothermal process with the conversion of water into hydrogen achieved by electrolysis. Because the entire feed stream must be electrolysed, the cost of electrolysis can result in a prohibitively expensive process for heavy water extraction and is practical only as a parasitic process where large scale electrolysis is performed for other reasons.
A second parasitic process is Combined Industrially Reformed hydrogen and Catalytic Exchange (“CIRCE”). CIRCE uses an industrial monothermal steam reformer for the first stage to generate hydrogen from methane and water feeds and electrolysis (typically CECE) for higher stages. Although it is more complex than the CECE process, the main attraction of the CIRCE process is the widespread availability of relatively large plants producing hydrogen by steam reforming. The CIRCE process suffers from the fact that elevated levels of deuterium in the reformer mean that leaks of any deuterated species (water, methane, hydrogen) from the reformer are particularly costly and the plant requires a high level of leak tightness in the reformer. Optimization of the CIRCE process for the lowest unit cost is primarily a balance between minimizing first stage catalyst volume and loss of deuterium with reformer leakage (by moving separative work into the higher stages) and minimizing electrolytic cell capital costs (by moving separative work into the first stage). However, as electrolytic cell capital costs dominate, the lowest unit cost solution is a distorted cascade with a first-stage enrichment five to seven times greater than in an ideal cascade, resulting in a configuration having high reformer losses and a comparatively low production. Indeed, the economics of the CIRCE process depend substantially on the cost of modifications required to make the industrial reformer relatively leak tight and on the amount of deuterium lost through the reformer. In addition, the CECE upper stages of the CIRCE process must receive a liquid (i.e. water) feed from the first stage.
An alternative process to harness water-hydrogen exchange is the Bithermal Hydrogen Water (BHW) process. BHW is a non-parasitic process using liquid phase catalytic exchange. In each stage there is an upper cold tower where the deuterium transfers from the hydrogen to the liquid water, and a lower hot tower where the deuterium transfers from the water to the hydrogen gas. The feed to the higher stages is taken from between the cold and hot towers. The BHW process is similar to the Girdler-Sulphide process, but with the advantages of much superior separation factors, lower energy consumption and non-toxic and non-corrosive process fluids.